This invention relates to the field of electronic instruments for measuring the polarization state of a beam of light and, more particularly, to such instruments that are capable of detecting effects on the polarization state of an incident light beam caused by an optical device under test (i.e., an optical system, subsystem, or component). Specifically, one embodiment of the invention provides a method and apparatus for impinging a light beam having predetermined states of polarization on an optical device under test to ascertain a response characteristic of the optical device to different polarization states and determining the polarization sensitivity of the optical device. One embodiment of the invention provides automatic polarization sensitivity determination to ascertain, for example, maximum and minimum transmission (or maximum and minimum reflection) of an optical device under test in response to the different possible states of polarization of an incident light beam, and the respective polarization states at which the maximum and the minimum transmission (or maximum and minimum reflection) occur.
There are various known techniques for measuring the polarization state of a light beam. The conventional technique for measuring the polarization state of a light beam is to align a waveplate and a linear polarizer in the optical path of the beam. The waveplate is configured to be rotatable about the optical axis, and is typically a quarter-wave plate. An optical sensor, such as a photodetector, is positioned downstream to measure the intensity of light transmitted by the waveplate and the polarizer.
In operation, the waveplate is sequentially rotated to a plurality of angular positions about the optical axis relative to the linear polarizer, and the transmitted light intensity is measured at each angular position by the photodetector. Intensity measurements at a minimum of four different angular positions are required for a determination of the state of polarization of the light beam. As is well-known, the polarization state of the light beam can be computed from these intensity measurements.
This technique has the drawback that it requires mechanical movement of the waveplate. Therefore, the speed of measurement of the polarization state is limited by the speed with which the waveplate can be rotated, and, in the case that the waveplate is rotated manually, measurement of the polarization state is time-consuming and inconvenient.
An apparatus that overcomes the above limitation is disclosed in U.S. Pat. No. 4,681,450 and in Azzam, R.M.A., et al., "Construction, Calibration, and Testing of a Four-Detector Photopolarimeter," Rev. Sci. Instruments, 59(1), January, 1988, pages 84-88. This apparatus comprises a series of four photodetectors serially located in the path of a light beam whose polarization state is to be measured. The light beam successively strikes each of the first three photodetectors obliquely, and is partially specularly reflected. Each specular reflection changes the state of polarization of the reflected light beam. Each photodetector produces an electrical signal proportional to the absorbed portion of the optical energy. The light beam is substantially fully absorbed in the fourth photodetector. The electrical signals produced by the four photodetectors can be used to calculate the Stokes parameters of the incident light beam, which determines the polarization state of the beam. Since this apparatus does not involve any mechanical movement, it does not have the speed limitation of the previously described apparatus or the inconvenience of a manual measurement.
The apparatus disclosed in U.S. Pat. No. 4,681,450 does, however, suffer from the drawback that the change in the polarization state of the light beam reflected at each of the photodetector surfaces is substantially wavelength-dependent. This apparatus must be calibrated by using four calibration light beams of different known polarization states. The calibration must be repeated for each different wavelength. Furthermore, at least one of the calibration light beams must not be linearly polarized; and such a beam is inconvenient to generate accurately. Calibration of the apparatus disclosed in U.S. Pat. No. 4,681,450 is, therefore, a formidable task. Consequently, the efficiency and accuracy of the apparatus is limited, particularly when polarization states of several different light beams are to be measured.
Another apparatus for performing polarization measurements is disclosed in U.S. Pat.No. 4,158,506. This apparatus, which is indicated to be suitable for measuring the polarization state of nanosecond optical pulses, comprises an assembly of six photodetectors positioned behind a corresponding assembly of linear polarizers and waveplates. A light beam passes through all of the linear polarizers simultaneously, and the transmitted light intensity from each polarizer is detected and measured by a corresponding photodetector. The electrical signals produced by the six photodetectors can then be used to determine the Stokes parameters of the incident light beam to indicate its polarization state.
Finally, another optical polarization measurement apparatus is disclosed in European Patent Application No. 8817382. In this apparatus, an incident light beam passes through a beam expander, and four separate portions of the beam pass through four coplanar Stokes filters. The four portions of the light beam are focused onto four associated photodetectors that measure the intensities of the received light. The electrical signals produced by the photodetectors are used to determine the Stokes parameters of the incident light beam to indicate its polarization state.
The apparatuses disclosed in U.S. Pat. No. 4,681,450 and European Patent Application No. 8817382 suffer from the same drawback, in that there is no provision for ensuring that the incident light beam whose polarization state is to be measured is properly aligned relative to the optical elements so that all photodetectors are subjected to the same beam. Furthermore, there is no provision for calibrating either apparatus. While European Patent Application No. 8817382 discloses an optical fiber input, and describes the phenomenon of "polarization noise" that results from transmission of a light beam through fiber optic material, no technique is disclosed for correcting for the polarization distortion of the input fiber.
Additionally, U.S. Pat. No. 4,306,809 describes apparatus having optical elements that are rotated automatically for determining the polarizing properties of a material on which a light beam impinges by measuring the Mueller matrix. However, neither this apparatus, nor the apparatuses described above, enable the polarization sensitivity of an optical device to be determined in response to various polarization states of an incident light beam.
In this regard, accurate characterization of optical devices is becoming increasingly important as optical devices become more complex and applications for optical devices proliferate, for example, in fiber optic telecommunications. One of the fundamental specifications of any optical device with an optical input and an optical output is polarization sensitivity, that is, the variation of optical power transmitted through the device (or reflected by the device) as the state of polarization incident on the input of the optical device is varied. For example, the splitting ratio and excess loss of a fiber optic directional coupler, the insertion loss of an optical isolator, and the gain of an optical amplifier all can exhibit variation as the input state of polarization is altered. In order to use such an optical device effectively in most practical applications, the polarization sensitivity of its transmission and/or reflection characteristics must be known.
Conventionally, polarization sensitivity of an optical device under test (DUT) has been directly measured by monitoring the output power of the optical DUT with a polarization-independent detector or optical power meter while the input state of polarization of an optical source is varied over all possible polarization states. This is a difficult and time-consuming technique.
Moreover, many arrangements have been devised to transform the fixed output state of polarization of an optical source into any desired state of polarization. Such arrangements are generally referred to as polarization controllers. For example, two independently rotatable quarter-wave plates constitute a polarization controller suitable for a light beam propagating through open space, and two or more single-mode optical fiber loops of variable orientation can serve as a polarization controller in fiber optic systems. See, LeFevre, H. C., "Single-mode fibre fractional wave devices and polarisation controllers," Elect. Lett., 16, 1980, pages 778-780. Both of these polarization controllers are manually driven and do not lend themselves to automation.
Alternatives for polarization controllers exist which can be electronically controlled. For example, polarization controllers based on stain-induced birefringence in an optical fiber, which is effected by piezoelectric or electromagnetic elements, have been demonstrated, as have polarization controllers based on electro-optic crystals or waveguides. See, Walker, N. G., and Walker, G. R., "Polarization control for coherent communications," J. Lightwave Technol., 8, 1990, pages 438-458. These polarization controllers are more easily automated, but an instrument employing any such polarization controller to measure polarization sensitivity has two fundamental disadvantages. One disadvantage is that the control inputs to a polarization controller do not correlate to the output state of polarization in an easily ascertainable manner, especially as the wavelength of the optical source varies. Moreover, the output intensity of the polarization controller is usually a weak function of the control inputs, and this variability in intensity translates directly into inaccuracy in the overall measurement. A second, more serious disadvantage is the necessity of a search algorithm. The state of a completely (not partially) polarized optical source has two degrees of freedom, so it is necessary to vary the state of polarization at the output of the polarization controller over a two-dimensional space while searching for the maximum and minimum transmission (or reflection).
Therefore, a method and apparatus for facilitating determination of polarization sensitivity of an optical device under test to various polarization states of an incident light beam are needed so that the response characteristic of the optical device to different polarization states can be assessed, for example, during the design of the optical device. Moreover, such a polarization sensitivity determination desirably would be calibrated, accurate, and rapidly obtained, as well as convenient to perform.